In electrical power distribution systems, distribution transformers are used to step down voltage between the high voltage power line and the user. Hence, transformers are present at virtually every distribution point. Transformers are typically mounted above ground on a junction pole, or at ground level on a pad or platform. The increased use of underground power distribution systems has resulted in a corresponding increase in the number of pad-mounted transformers. Pad-mounted transformers must be contained in a manner that will prevent tampering by unauthorized individuals. Typically, this is accomplished by enclosing the high voltage and low voltage termination components of the transformer within a metal cabinet. The transformer includes a tank, which contains the core and coil assembly immersed in oil, and a cabinet, which includes a top hinged door and a bolted in place sill. Connections for incorporating the transformer assembly into the power distribution system extend through one wall of the tank and are enclosed by the cabinet. In order to provide utility personnel the necessary access to the transformer connections the cabinet must include a door. Conventionally, such cabinet doors are three-dimensional, comprising a front, two sides and a top and are typically formed in two pieces, which are welded together. The cabinet door is pivotally attached to the tank along the top edge of the front plate of the tank. The cabinet also includes a low sill extending forward from the transformer tank, upon which the cabinet door rests when closed.
As a result of this construction, a tamper proof interface must be provided along the three rear edges (two sides, top) where the door interfaces with the tank, and along the three lower edges (two sides, front) where the door interfaces with the sill. The interface must be able to withstand the attempted insertion of wire probes or other objects. In addition, the cabinet door must be rigid enough to withstand attempts to pry open the cabinet. Standards for tamper and pry resistance are set by ANSI.
Conventional transformer cabinet doors include a plurality of plastic or metal tamper strips mounted along the inside of the edges of the door. The strips are often mitered and notched so as to fit together and form a continuous tamper strip around the inner edge of the door. The purpose of the tamper strips is to prevent wire probes or other foreign objects from being inserted into the space between the door and the tank or sill and, to a lesser extent, to stiffen or strengthen the door so that it resists efforts to pry it open.
Conventional tamper strips come in a kit comprising one each of several linear strips. The strips are not interchangeable and each strip must be mounted in the proper position and in the proper order on the cabinet door. Plastic strips are affixed to the door by means of welded on studs or bolts and hardware comprised of speednuts or flat and lock washers and hex nuts. Metal strips may be affixed by tack welding. Metal strips may be affixed to the door prior to painting, while plastic strips are typically applied after the cabinet door has been assembled and painted. The mounting of the strips on the cabinet door comprises a significant additional manufacturing step in terms of labor and time. The hardware or welds that are used to affix the strips to the cabinet door form initiation points for corrosion. Also, the interface between the strips and the door to which they are attached forms a small gap into which water can be drawn by capillary action, resulting in increased corrosion. Furthemore, if the strips are tack-welded this gap may be difficult to paint.
U.S. Pat. No. 4,559,699 to Owen et al. discloses a cover/tank interface that consists of two equal opposite facing acute angles in the cover and a single obtuse angle in the adjacent tank, with the obtuse angle being substantially equal to 180.degree. minus the value of one acute angle. Closure of the terminal cover causes a leg of the obtuse angle to be received in the notch formed by the second acute angle. Fabrication of such a cover/tank interface is difficult, however, as close tolerances must be maintained around the entire interface. Also, the "V" shaped grooved formed in this manner is difficult to coat with paint or other protective coating.
Hence, it is desired to provide a means for preventing prying of the door with respect to the sill and tank or the insertion of foreign objects into the interface therebetween, which does not require an additional manufacturing step or the affixation of non-metal parts to the metal door. It is further desired to provide a cabinet door that is easily manufactured yet forms an interface with the sill and tank and meets the tamper-proofing standards required by ANSI.